Both anterior and posterior VBI are dependent on BMP signaling for erythropoiesis. (A-D) Embryos were injected into dorsal (A,C) or ventral (B,D) blastomeres at the 4-cell stage with 0.8 ng/embryo of ER-xSmad6 mRNA and cultured in the absence (A-B) or presence (C-D) of E2 from stage 10.5. Embryos were fixed and stained with benzidine (blue stain) at stage 35. Arrowheads indicate approximate location of liver primordium and lines indicate extent of ventral blood island staining. Original magnification, × 20.